高温变形测量中热流场造成畸变的修正方法

目前高温环境中材料的变形测量是研究的热点,基于数字图像特征识别的非接触测量方法促进了高温环境变形测量的发展,但由于高温环境的复杂性,存在很多测量影响因素,其中高温环境中热流场的存在对数字图像法的影响尤为明显。本文提出了一种对高温变形测量中热流场造成畸变影响的修正方法。针对基于光学成像方法的材料高温变形测量中常见的热流场扰动,通过数值仿真得到热流场模型及热流场分布状况,再结合光线追迹原理对热流场造成的图像畸变影响进行分析,用数值分析结果对高温变形测量实验结果进行修正。对比扰动修正结果与真实位移有很好的一致性,从而证明了所提方法的可行性和有效性。     

关于用扰动速度图法求解对称楔超空泡绕流的一个注记

本文给出了文献[1]中对称楔超空泡绕流的阻力系数计算公式。此公式与平板绕流的试验结果[2]及Lindsey[3]的对称楔的试验结果非常吻合。     

Active feedback control of stall in an axial flow fan under dynamic inflow distortion

This work proposes an active feedback control strategy using cross-correlation technique in a single stage axial flow fan operating under dynamic inflow distortion. Experiments were carried out under dynamic inflow distortion at the design speed with only three sensors and actuators, each. The stall inception mechanism studies under dynamic inflow distortion were carried out using 1-D continuous Morlet wavelet transform. It was observed that stall inception under co- (in the same direction of rotor rotation) and counter-rotating (in the opposite direction of rotor rotation) inflow distortion occurred through long and short length-scale disturbances, respectively. The knowledge of the nature of instabilities under dynamic inflow distortion was used to set the threshold of the correlation coefficient. It was observed that the active feedback control strategy resulted in a stall onset delay of 125 (?3.125 s) and 65 (?1.625 s) rotor revolutions under co- and counter-rotating inflow distortions, respectively. The highest delay under co-rotating inflow distortion was attributed to the substantially higher stall warning time as compared to counter-rotating inflow distortion.     

Pressure spectra in homogeneous low Reynolds number turbulent shear flow subjected to weak rotation

In this paper, pressure spectra have been derived from the authors’ model (Eur. J. Mech., B/Fluids 12 (1) (1993) 31–42) developed by means of rapid distortion theory (RDT) of homogeneous low Reynolds number turbulent shear flow subjected to weak rotation. The combined effects of uniform shear dU₁/dx₂ and weak rotation Ω₃ on the evolution of pressure spectra have been examined in terms of the rotation number 2Ω₃/(dU₁/dx₂). It is found that the system rotation exhibits the opposite effect on the pressure field as compared with the influence of rotation on the velocity fluctuations.     

The significance of pure measures of distortion in nonlinear elasticity with reference to the Poynting problem

The objective of this paper is to show the significance of expressing the strain energy function in terms of a scalar pure measure of dilatation and a tensor pure measure of distortion, which were essentially introduced by Flory [1]. It is shown that convenient representations for the strain energies of dilatation and distortion, and the pressure and deviatoric Cauchy stress may be recorded in terms of these deformation measures. After specializing to the case of an isotropic material, specific constitutive equations are proposed and the Poynting problem is considered. It is shown that the Poynting effect (extension of a bar in torsion) is significantly influenced by coupling between dilatational and distortional strain energies, which is caused by the dependence of the shear modulus on dilatation.     

Superposition principles for small amplitude oscillatory shearing of nematic mesophases

The linear viscoelasticty of Leslie-Ericksen monodomain liquid crystals subjected to a bend distortion through a small amplitude oscillatory shear flow driven by harmonic wall stress is analyzed, using numerical and asymptotic methods. The viscoelastic material functions were derived using a new scaling approach that extracts the material parameters that control superposition. Small and high frequency superposition schemes for linear viscoleasticity were derived. The schemes were successfully applied to collapse the predicted loss and storage linear viscoelastic moduli of seven experimental data sets. Comparisons between different shear flows (simple shear and capillary Poiseuille) and different director distortion modes (splay and bend) shows that the superposition schemes are applicable to shear flows in any single director distortion mode.     

Invariant solutions in a channel flow using a minimal restricted nonlinear model

Simulations using a Restricted Nonlinear (RNL) system, where mean flow distortion resulting from Reynolds stress feedback regenerates rolls, is applied in a channel flow under subcritical conditions. This quasi-linear restriction of the dynamics is used to study invariant solutions located in the bulk of the flow found recently by Rawat et al. (2016) [14]. It is shown that the RNL system truncated to a single streamwise mode for the perturbation supports invariant solutions that are found to bifurcate from a relative periodic orbit into a travelling wave solution when the spanwise size is increasing. In particular, the travelling wave solution exhibits a spanwise localized structure that remains unchanged for large values of the spanwise extent as the invariant solution lying on the lower branch found by Rawat et al. (2016) [14]. In addition, travelling wave solutions provided by this minimal RNL system are self-similar with respect to the Reynolds number based on the centreline velocity, and the half-channel height varying from 2000 to 5000.     

ON THE STABILITY OF DISTORTED LAMINAR FLOW(Ⅱ)——THE LINEAR STABILITY ANALYSIS OF DISTORTED PARALLEL SHEAR FLOW

Based on the hydrodynamic stability theory of distorted laminar flow and the kind of distortion profiles on the mean velocity in parallel shear flow given in paper [1], this paper investigates the linear stability behaviour of parallel shear flow, presents unstable results of plane Couette flow and pipe Poiseuille flow to two-dimensional or axisymmetric disturbances for the first time, and obtains neutral curves of these two motions under certain definition.     

Pressure losses in transitions between square and rectangular ducts of the same cross-sectional area

Experimental results are presented for the pressure loss in transitions between square and rectangular ducts where the two ends have the same cross-sectional area. The aspect ratios at the rectangular end ranged from 0.3 to 0.625, and the transition length from 1 to 2 times the hydraulic diameter. Reynolds numbers ranged from 50 000 to 125 000. The pressure drop may be divided into components arising from friction and velocity profile distortion. The friction component, which may be evaluated by normal pipe flow methods, accounts for the observed variation with Reynolds number. The velocity profile component increases as the aspect ratio of the rectangular end falls, and is significantly higher for rectangular to square than for square to rectangular transitions. There is an optimum length to hydraulic diameter ratio, for which the pressure loss is a minimum; it has not been found exactly, but is less than 2 and probably below 1.     

Second-order effect of an elastic circular shaft during torsion

In this paper we deal with the second-order effect of an elastic circular shaft duringtorsion.The analysis is based on the method of co-moving coordinates and the strain-rotation decomposition theorem in continuum mechanics.By using asymptotic expansionmethods,we comfirm that the effect of axial elongation and distortion of plane cross-section exists in an elastic circular shaft during large torsion and give the expressions ofthe axial force and the torque.     

The effect of non-linear interaction between gas and particle velocities on the hydrodynamic stability in the Blasius boundary layer

A weakly non-linear stability analysis of two phase flow in the Blasius boundary layer has been carried out. Two mathematical models have been established based on the perturbation shape preserved assumption and linear stability model of two phase flow proposed by Stuart [On the non-linear mechanics of hydrodynamic stability, J. Fluid Mech. 4 (1958) 1-21] and Saffman [On the stability of laminar flow of dusty gas, J. Fluid Mech. 13 (1962) 120-128], respectively. The perturbation model and the perturbation energy balance equation are solved numerically with Chebyshev spectral method and artificial boundary condition. The numerical program adopted in the present study is verified by comparison with former works. The results show that the non-linear interaction between mean flow and perturbation reduces the growth rate of perturbation, while the non-linear interaction between particle phase and gas phase increases the growth rate of perturbation amplitude. The distortion of the mean flow caused by the Reynolds stress modifies the rate of transfer of energy from the mean flow to disturbance. The existence of particle alleviates the distortedness. The result also indicates that the weakly non-linear stability theory is consistent to linear stability theory, and the addition of fine and coarse particles reduces and increases the critical Reynolds number.     

On the modeling of fluidity loss phenomena in Couette and Poiseuille flows of elastic liquids

Some effects of the possible relaxation transition from viscoelastic liquid state to highly elastic solid state were theoretically and numerically investigated in the shear situations, within the approach proposed in papers [1, 2, 5, 16]. It was found that for a single Maxwellian model the constitutive equations developed in [1, 2, 5] are not valid at elevated shear stresses. Some new aspects of the possible rheological behavior of elastic liquids in subcritical (before transition) and supercritical (after transition) regimes were demonstrated. The mechanism of fluidity loss studied in this paper could serve as a possible trigger mechanism for the melt flow instabilities.     

HARMONIC SOLUTIONS OF SOME SECOND-ORDER NONLINEAR EQUATIONS UNDER A PERIODIC FORCE

In this paper we prove some theorems on the existence of harmonic solutions of somesecond-order nonlinear equations under a periodic force.These theorems extend relevantresults in refs.[1]-[8].     

在波形斜坡上的自由粘性流动的算法

利用非定常变换将流体流动的复杂区域变到一个固定的矩形计算区域，然后用高效的数值方法对此强非线性问题进行积分．由于变换本身是随自由面变化而变化，刻画这一变换的参数将作为未知量与其它待求流动参数在积分过程中同时求得．本方法对流动问题不作任何限制，可以计算非常复杂的自由粘性流动问题．     

On the stability of distorted laminar flow(III)—The nonlinear stability analysis of distorted parallel shear flow

Based on the hydrodynamic stability theory of distorted laminar flow and the kind of distortion profiles on the mean velocity in parallel shear flow given in paper [1], this paper investigates the nonlinear stability behaviour of parallel shear flow, carries on stability calculation taking account of the perturbations of background turbulence noise under certain assumption, and obtains some results in accordance qualitatively with those of experiment for plane Poiseuille flow and pipe Poiseuille flow.The author thanks Prof. Zhou Heng sincerely for his kind offer of his computer program of the artificial neutrality method on the stability in subcritical range of plane Poiseuille flow.     

On perfectly stress field at a mixed-mode crack tip under plane and anti-plane strain

In [1], under the condition that all the perfectly plastic stress components at a crack tip are functions of ϕ only, making use of equilibrium equations, stress-strain rate relations, compatibility equations and yield condition. Lin derived the general analytical expressions of the perfectly plastic stress field at a mixed-mode crack tip under plane and anti-plane strain. But in [1] there were several restrictions on the proportionality factor γ in the stress-strain rate relations, such as supposing that γ is independent of ϕ and supposing that γ=c or cr⁻¹. In this paper, we abolish these restrictions. The cases in [1], γ=cr^d (n=0 or-1) are the special cases of this paper.     

An endochronic theory accounting for deformation induced anisotropy of metals under biaxial load

Using the experimental results of yield surfaces obtained by Wu and Yeh [1991] (Int. J. Plasticity, 7, 803) for 304 stainless steel, this work provides a verification of the endochronic theory of plasticity accounting for deformation induced anisotropy. The experiments were performed under proportional loading conditions. The main difference between this paper and other papers that attempt to describe the distortion of a yield surface is that, in addition to distortion, motion of yield surface (kinematic hardening) has also been addressed by this paper. The result has shown that the theory predicts the experimental data with substantial accuracy. However, since in this theory the plastic strain increment, although normal to the initial yield surface, is in the radial direction emanating from the center of the subsequent yield surface, validity of the present model must be further studied for the case involving nonproportional loading conditions.     

The Calculation of Semi-circular Corrugated Tube——An Application of the General Solution of Ring Shell

In this paper, the deformation and stress distribution in semi-circular corrugated tube under axial force are calculated by means of the general solutions of circular ring shell given in previous paper[1].     

Effect of external radiation on the distortion of a vitreous body in a flow of gas

The semitransparency of a material, which determines the penetration of external radiation into the inside layers, affects significantly the temperature profile in the body [1, 2]. Since the viscosity of a melt of viscous materials depends strongly on the temperature, deformation of the temperature profile close to the surface leads to considerable change of the rate of spread of the liquid film, which has a significant effect on the rate of distortion of the body. In the present paper, the problem of distortion is formulated taking into account the transfer of radiation inside the body. The dependence of the distortion parameters and the degree of blackness of the body on the fraction of radiation in the external thermal flux and the mean free path of the radiation inside the material is determined. A sufficient condition is also obtained for the presence of a temperature maximum inside the body in a more general case than in [3].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 127–134, July–August, 1976.The author thanks G. A. Tirskii for a discussion of the posing of the problem.     

集中力作用下的两相饱和介质位移场Green函数

以复模两相饱和介质Biot动力学方程为基础,根据该方程D'Alembert解的Fourier变换所属的Homholtz方程特性,由Biot方程解的相容性条件及δ函数性质较好地解决了快、慢纵波位势的耦合问题.较为简便地得到了两相饱和介质在集中力作用下低频(ω＜ωc)时的频域和时域的Green函数.